A huge one for me is the desk. Height adjustable chairs are standard. Height adjustable desks need to be also - you are unlikely to be one of the rare humans with exactly the right torso/arm size for the standard desk height.

The standard height was normalised when everyone wrote with a pen - it's just too high for typing even for me and I'm very tall. Look up the proper arm/wrist/head angles, you'll see it's not possible to achieve unless your feet are on a tall footrest and your chair goes higher than the standard office chair height. It's far more practical to lower the desk than raise your feet/butt.

Quality electric height adjustable desks are affordable these days, so aside from that benefit you can also spend some of your time standing up (don't stand all day - also start with short stints and build up to longer ones).

Monitor height is important too. I've never found a monitor that comes with a stand that is high high enough for me and laptops are obviously way too low. VESA arms are best - but books work too. The main advantage for VESA over a stack of books is you can put other stuff where the books are.

I personally don't think ergonomic keyboard/mice are the way to go. They tend to force you to keep your arms/hands in exactly the same position. It might be a comfortable position but it's still no good to repeat exactly the same muscle movements over and over.

Instead, go with a (good) regular keyboard, maybe a split one but not a sculpted one. Your hands should "float" while typing with just your finger tips touching the keys, don't let your palms touch anything - your arms should be moving while typing not fixed in one spot. It will take time to get used to this, but it's worth it.

For the mouse... learn as many keyboard shortcuts as you can and get yourself a desktop trackpad, put it where laptop trackpads are, for scrolling, gestures, etc. You won't have RSI issues form mouse use if all you do is click on things where you actually need to do that.

If you're going to a doctor... be careful. Most of them really don't know anything about this issue and unless they're a friend or family they likely won't be willing to spend the time necessary to research it just for you. I'm not saying don't go to a doctor but make sure you find a specialist and verify anything they tell you. I've received some really bad advice from doctors.